This invention relates to turbofan engines utilizing a mechanical constant speed drive and particularly to the cooling system for the lubricant used in the constant speed drive.
As is well known the constant speed drive, which is a gear and clutch arrangement, serves to generate electricity for the aircraft. In a turbofan driven aircraft, the fan air/lubricant cooler is located in the fan duct (as shown in FIG. 2) and extends into the fan airstream. Associated with this type of plate/fin heat exchanger is a pressure loss which is reflected in terms of aircraft performance penalty. In a given installation utilizing the JT-9D engine (manufactured by the P&WA division of UTC, the assignee) this pressure loss amounted to approximately a loss of 0.8% of TSFC (thrust specific fuel consumption).
It has also been well known that a typical constant speed drive lubricant cooling system would use a single heat exchanger which is of the plate/fin type in the fan airstream as described above.
We have found that we can improve TSFC by reducing the size of the fan air/oil heat exchanger because fan stream pressure losses have been reduced by utilizing the engine/oil heat exchanger that is already in existence. When the JT-9D was upgraded to increase its thrust, a doubling in size of the existing fan air/oil cooler would have been necessary. By virtue of this invention, the size of the cooler on the upgraded engine was actually reduced in size by a factor of 6, or approximately 1/3 of the predecessor engine cooler. In terms of TSFC, an improvement of 1% was realized.
Moreover, there are advantages gained from utilizing the engine fuel/oil cooler that wasn't available heretofore. Namely, because of fuel pump inefficiency and engine oil heat transfer, a large temperature rise of the fuel is occasioned during aircraft descent. This is primarily due to the pilot cutting back on the power lever reducing thrust and engine power, which causes the fuel to recirculate resulting in a higher fuel temperature. Connecting the CSD lubricant to the engine fuel/oil heat exchanger now serves to reduce the temperature of the fuel prior to it being admitted to the engine's combustor. This lower fuel temperature, in effect, reduces the adverse effect of the higher temperature fuel during descent on the combustor, resulting in a longer life of the combustor.
By locating the CSD fuel/oil portion of the engine fuel/oil cooler in a downstream position, relative to fuel flow, the heat transfer from the CSD lubricant does not interfere with the engine lubrication system.